It is well-known to measure temperature along an optical fibre by means of Fibre Bragg Gratings. According to prior art, a plurality of Fibre Bragg Gratings are inscribed along an optical fibre. Each Fibre Bragg Grating is active in its particular wavelength range within the electromagnetic spectrum and reflects a certain narrow wavelength band, within the particular wavelength range, depending on parameters like temperature, pressure, strain and indices of refraction. The reflected narrow wavelength band has a peak at the Bragg wavelength. A temperature change results in a shift of the peak wavelength. In order to avoid uncertainties regarding which Fibre Bragg Grating that contributes to a certain reflected peak wavelength, each Fibre Bragg Grating is given its unique wavelength range. Since a light source has a limited total wavelength range, only a limited number of Fibre Bragg Gratings with a unique wavelength range can be inscribed along each optical fibre.
The disadvantage with having a limited number of Fibre Bragg Gratings is that the number of measurement points may be small in relation to the length of the optical fibre. Therefore there is a risk to miss areas, between measurement points where a temperature change occurs. U.S. Pat. No. 6,204,920 solves the problem of having a limited number of measurement points by implementing further optical fibres, wherein the Fibre Bragg Gratings of the further optical fibres uses the same wavelength ranges as the Fibre Bragg Gratings of the first optical fibre. The sensor system is able to distinguish from which of the optical fibres that light has been received.
There is a need for a sensor system where further Fibre Bragg Gratings can be inscribed along one optical fibre without increasing the complexity of the system.